Changes in the circulating form of serum somatomedin-C during fetal life.

The predominant form of somatomedin-C (Sm-C) in human postnatal serum elutes from gel chromatographic columns at an approximate molecular weight of 150,000 daltons (150 K). Nine of 10 infants of 30 weeks gestation or more had elution profiles similar to postnatal sera, while in 7 of 9 infants of 27 weeks or less, the immunoreactive Sm-C eluted predominantly at an apparent molecular weight of 40,000 daltons (40 K). Five infants between 26-32 weeks exhibited a transitional pattern. One 43 week gestation anencephalic infant exhibited only 40 K Sm-C, suggesting that the 150 K proteins which bind Sm-C are acquired in response to growth hormone or other pituitary hormones. Even though the 40 K form of Sm-C is the only form found in mid-gestation fetal serum and in media from in vitro fetal mouse liver explants, it probably does not represent the primary gene product. This is suggested by the observation that 40 K Sm-C also binds 125-I-Sm-C and can be dissociated by acid and, therefore, probably is a complex of Sm-C non-covalently bound to other proteins.